1. Field of the Invention
The present invention relates to a method of preparing nano-level Pt/C electrocatalyst for cathode of fuel cell.
2. Description of the Related Art
Polymer electrolyte membrane fuel cells (PEMFC) have the characteristics of high energy transfer efficiency, low pollution to environment, mild operating condition, small volume, right weight, safety in use and long lasting. They are comparatively suitable to be used as power source for vehicles and portable electrical source. At present, much attention is focused on the researches of PEMFC by many countries in the world. Reduction of oxygen is the reaction taken place at the cathode of PEMFC. The performance of this reaction will directly influence the voltage and output power of the whole cell, and the performance of the cathode depends mainly on the catalytic properties of the cathodic-electrocatalyst. Up to now, the cathodic electrocatalysts which have been studied include mainly the following: (1) platinum and its alloys; (2) macrocyclic compounds of transition metals, especially those of porphyrin and phthalocyanine compounds having Fe or Co as the central metal ion; (3) transition metal oxides having the structures of perovskite, pyrochlore, etc.; (4) transition metal cluster compounds. However, the intrinsic catalytic activity and stability of the last three kinds of catalysts are lower in comparison with those of platinum and platinum alloys. Therefore, cathodic electrocatalysts now in practical use for PEMFC are mainly those of Pt/C.
In Pt/C electrocatalyst, the size of platinum particle is the main factor influencing the catalytic activity for oxygen reduction. Peukert showed that the specific activity per unit mass of Pt/C electrocatalyst, when particle size of platinum is in the range of 3 to 5 nm, is the highest (M. Peuckert, et al. 1986 J Electrochem Soc 133:944-947). The particle size of platinum depends on the method of preparation of the Pt/C electrocatalyst. At present, there are mainly two kinds of method for preparing the Pt/C electrocatalysts. One is called sol-gel method. In this method, chloroplatinic acid is first converted into complex compound of platinum, which is then used to prepare Pt/C electrocatalyst (H. Bonnemann, et al. 1991 Angew Chem 103:1344). Said method imposes comparatively strict demands on temperature, concentration of solution, pH value and reaction time. The other one is called impregnation method. Starting directly from chloroplatinic acid, Pt/C electrocatalyst is prepared through different methods (J. B. Goodenough, et al. 1990 Electrochimica Acta 15:199-207). In the latter method, chloroplatinic acid solution is diffused into the interstices of active carbon due to capillary action, adsorbed on the active carbon and reduced by reducing agent. As a result, the adsorption capability of chloroplatinic acid will have significant influence on the performance of final catalyst. After the addition of reducing agent, chloroplatinic acid in the bulk solution will be reduced first owing to the existence of hindrance to diffusion. On the other hand, because there exists adsorption equilibrium between chloroplatinic acid adsorbed on active carbon and that in bulk solution, and the adsorption equilibrium will move towards the direction of chloroplatinic acid in bulk solution at this moment, part of the chloroplatinic acid adsorbed on the interstices of the active carbon will be desorbed and will enter the bulk solution. Thus in practice, most of the reduction of the chloroplatinic acid is carried out in the bulk solution. The catalyst obtained by means of such kind of reduction will suffer from the coagulation of platinum particles, lowering of homogeneity of platinum particles and poor supporting on active carbon. It is generally very difficult to prepare nano-catalyst with platinum micro-particles homogeneously distributed on the interstices of the active carbon as well as on the surfaces of the active carbon.
The objective of the present invention is to provide a method for preparing nano-level Pt/C electrocatalyst for cathode of fuel cell. The method of the present invention can control the adsorption of chloroplatinic acid on active carbon by adding an anchoring agent, which can precipitate the chloroplatinic acid entering the interstices of active carbon in the form of ammonium chloroplatinate or potassium chloroplatinate, thereby inhibiting desorption of chloroplatinic acid from the interstices of active carbon and entering of them into the bulk solution. Consequently, Pt/C electrocatalyst of homogeneous particle size with platinum micro-particles homogeneously distributed in the interstices as well as on the surfaces of the active carbon is obtained. The platinum particle size in the catalyst is mainly in the range of 2.5 to 4.5 nm. The specific activity per unit mass of the said electrocatalyst for catalyzing the reduction of oxygen is high and thus is suitable for use as the catalyst of fuel cell cathode.
According to one aspect of the present invention, a method for preparing the nano-level Pt/C electrocatalyst for cathode of fuel cell is provided. Said method comprises the following steps:
(1) dispersing active carbon in a mixed solvent composed of organic solvent and deionized water to form a suspension, wherein the content of active carbon in the mixed solvent is in the range of 0.1 to 1 g/L;
(2) adding an aqueous solution of anchoring agent into the suspension obtained in step (1) to make anchoring agent adsorbed on the active carbon, wherein the mole ratio of the anchoring agent to chloroplatinic acid which is to be added in the following step is in the range of 3:1 to 6:1;
(3) adding chloroplatinic acid solution into the above mixture to allow chloroplatinic acid adsorbed on the active carbon, wherein the concentration of chloroplatinic acid solution is in the range of 0.1 to 1 g/L, based on the weight of platinum; and
(4) adding reducing agent to yield platinum particles after reduction to obtain Pt/C electrocatalyst, wherein the temperature of reduction is in the range of 0 to 80xc2x0 C.
According to another aspect of the present invention, a method for preparing the nano-level Pt/C electrocatalyst for cathode of fuel cell is provided. Said method comprises the following steps:
(1) dispersing active carbon in a mixed solvent composed of organic solvent and deionized water to form a suspension, wherein the content of active carbon in the mixed solvent is in the range of 0.1 to 1 g/L;
(2) adding an aqueous solution of anchoring agent into the suspension obtained in step (1) to make anchoring agent adsorbed on the active carbon, wherein the mole ratio of the anchoring agent to chloroplatinic acid which is to be added in the following step is in the range of 3:1 to 6:1;
(3) adding chloroplatinic acid solution into the above mixture to allow chloroplatinic acid adsorbed on the active carbon, wherein the concentration of chloroplatinic acid solution is in the range of 0.1 to 1 g/L, based on the weight of platinum;
(4) adjusting the pH value of the above mixture to the range of 5.5 to 10.5 using ammonia water or potassium hydroxide solution; and
(5) adding reducing agent to yield platinum particles after reduction to obtain Pt/C electrocatalyst, wherein the temperature of reduction is in the range of 0 to 80xc2x0 C.
In the method of the present invention, active carbon is firstly dispersed in a mixed solvent to form a suspension with its content in the suspension being from 0.1 to 1 g/L. Said mixed solvent is composed of a polar organic solvent and deionized water. The volume ratio of the organic solvent to deionized water can be in the range of 1:10 to 10:1. The examples of organic solvents suitable to be used in the present invention include, but are not limited to, methanol, ethanol, isopropanol, diethyl ether and acetone.
In the method of the present invention, anchoring agent is first loaded to active carbon carrier before loading chloroplatinic acid. The term xe2x80x9canchoring agentxe2x80x9d, used in this invention, means compound which can, in a form of suitable salt, deposit the successively added chloroplatinic acid onto the active carbon carrier.
The anchoring agent useful in the present invention can be selected from the group consisting of ammonium chloride, potassium chloride, ammonium bromide, potassium bromide, ammonium iodide, potassium iodide and combination thereof.
Said anchoring agent is generally added in the form of aqueous solution. In principle, there is no limitation to the concentration of anchoring agent solution. For example, the concentration of anchoring agent solution could be from about 1 wt % to saturated concentration, but for the sake of easy operation, the concentration of the aqueous solution of anchoring agent can be generally in the range of 10 to 60 wt. The mole number of the anchoring agent of the present invention is in the range of 3 to 6 times that of the successively added chloroplatinic acid.
Optionally, after the loading of chloroplatinic acid, the pH value of the reacting solution can be adjusted to the range of 5.5 to 10.5 by either ammonia water or potassium hydroxide. Then reducing agent is added. Alternatively, after the loading of chloroplatinic acid, reducing agent in excess amount can directly be added into the reacting solution. The examples of reducing agent suitable to be used in the present invention include, but are not limited to, formaldehyde, formic acid, hydrazine hydrate, sodium borohydride and potassium borohydride. The mole number of reducing agent used is 3 to 6 times that of chloroplatinic acid. The temperature of the reduction reaction can be in the range of 0 to 80xc2x0 C.
After the reduction reaction is completed, conventional processes of filtration, washing and drying are used to yield nano-level Pt/C electrocatalyst of the present invention. For example, Pt/C electrocatalyst could be obtained after vacuum drying for 1 to 6 hr at the temperature of 80 to 100xc2x0 C.
According to the present invention, Pt/C electrocatalyst for cathode of fuel cell can be obtained at lower cost and by more simple process and has higher catalytic activity in comparison with those electrocatalysts obtainable by methods of prior art.
The Pt/C electrocatalyst obtainable by the present method can be used as active component in the cathode of fuel cell, especially in the cathode of polymer electrolyte membrane fuel cells.